The invention relates to a solid state pick-up camera having a photosensitive target. The target comprises a semiconductor body having a surface at which there are a number of charge storage capacities. During operation charge carriers of a first type, which are generated locally in the semiconductor body by absorption of incident radiation, can be stored and integrated in the storage capacities for a certain integration period. The photosensitive target further includes means for preventing excess charge carriers, which are generated during local overexposure, from spreading in the semiconductor body from a full charge storage capacity to adjacent storage capacities.
The photosensitive target may be, for example, of the charge-coupled type which is generally known from published literature. Such a device comprises a series of clock voltage electrodes which are situated one behind the other. During operation clock voltages can be applied to these electrodes so that in the charge transport channel (below the clock voltage electrodes) a train of charge packets can be moved to a read-out member and can be read out there sequentially. The charge packets can be generated in the CCD itself and can be stored by inducing in the CCD for a certain integration period a pattern of potential wells which are separated from each other by barriers and which are accessible to incident radiation. In this manner a radiation pattern can be converted into a pattern of charge packets which have been obtained by integration of the locally generated photocurrents in the potential wells. After termination of the integration period the charge packets can be transported to a read-out member in a charged-coupled manner.
A known problem in this type of target occurs in the case of overexposure, in which such a large excess of charge carriers is formed that charge from a full potential well diffuses to adjacent, not yet full potential wells. This phenomenon, often referred to as "blooming", in charge-coupled devices has the tendency of filling in the first instance the potential wells in the same CCD line. Thus, a pinpoint overexposure in displaying the picked up picture will result in a very annoying white line extending vertically over the picture.
In the article entitled "Blooming Suppression in Charge-Coupled Area Imaging Devices" by C. H. Sequin (published in The Bell System Technical Journal of October 1982, pp. 1923-1926), a photosensitive target is described in which drain channels which are separated from the CCD channels via overflow barriers are provided between the CCD registers. Since the overflow barriers are slightly lower than the potential barriers between successive potential wells in the CCD channels, excessive charge carriers, before spreading over a CCD channel, can be removed via the overflow barriers and the drain channels.
This method very effectively prevents "blooming". However, due to the presence of the drain zones and the means to form the overflow barrier, the sensor will become larger and more complex. In addition, the parts of the semiconductor body present between the CCD and occupying a comparatively large part of the photosensitive surface do not contribute to the photosensitivity, so that the advantage of eliminating blooming is offset by a decrease in the resolving power and sensitivity of the device.
In the article entitled "Control of Blooming in Charge-Coupled Imagers" by W. F. Kosonocky et al (R.C.A. Review, Vol. 35, March 1974, pp. 3-24), an "anti-blooming" method is described when using a CCD with a surface channel. In this method, the surface regions are brought into an accumulation mode during the integration period at the area of the potential barriers between the potential wells, that is to say they are brought into a state in which an excess of charge carriers of the other type (opposite to the charge carriers of the photocurrent) is present.
This method has the advantage that the sensor is not made larger or more complex. However, the effectiveness of this method will be lower than that of the first method, since a certain amount of blooming will nevertheless occur due to diffusion. Moreover, this method can be used only in CCD sensor with surface channels.
It is to be noted that the phenomenon of "blooming" can also occur in devices other than the ones described here. For example, blooming can also occur in charge-coupled sensors of the interline type or in so-called xy-sensors in which the reading out takes place by selecting the photoelements in the x-direction and the y-direction. Although the invention (to be described below) is of particular importance for the above-described pick-up sensors in which the generation and integration of photocurrents take place in the charge coupled device itself, the discoveries on which the invention is based can in principle be applied also to pick-up cameras having other photosensitive targets in which the phenomenon of blooming may occur.